-ance or -ence
In an unstressed syllable the two endings sound the same. The way through is a related word that keeps the vowel.
The rule
-ance and -ence flatten to the same sound in an unstressed syllable, so the ear cannot settle it. A relative often can: -ant goes with -ance and -ent with -ence (important, importance).
Words in -ance usually have relatives in -ant, and words in -ence relatives in -ent: important, importance; relevant, relevance; confident, confidence; different, difference. Whichever of the pair you are surer of settles the other.
The verb behind a noun often leans one way: appear, assure and comply give -ance, while adhere, differ and exist give -ence. A tendency worth knowing, not one to trust.
A soft C or G before the ending usually takes -ence (innocence, intelligence, negligence), and a hard one -ance (elegance, significance).
Where it stops holding
Where neither a relative nor the consonant helps, the ending is learned with the word, as in maintenance and independence.
Background
Settling an ending from a related word is reasoning from meaning, which the research on word structure finds worth teaching. That research is on how people learn to spell.
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